Tree defoliators, namely insects, can cause serious damage when outbreaks are severe. In the urban environment, such outbreaks are considered to be more common, because of the urban heat island effect – this is because insects are ectotherms (they require an external heat source). Insects will, in theory, therefore fare much better in such urban environments, compared to nearby rural locations. Additionally, as the vigour of trees will change as a result of warmer urban climates, such alterations may have a positive (or negative) impact upon insect populations. For example, trees that fare less optimally may produce fewer secondary metabolites that dissuade defoliation, and emit fewer herbivore-induced plant volatiles that disrupt herbivory and attract predators and parasites of the insect defoliator. Of course, this has consequences for the host trees, as defoliation outbreaks may thus be more severe, more prolonged, more frequent, and thus more damaging.
In order to add weighting to this statement, the authors of this study investigated how the oak scale insect Parthenolecanium quercifex fared in urban environments across Raleigh, North Carolina, USA, with a specific focus upon how urban temperatures influenced their abundance both directly (attraction to warmer areas, increased fecundity in females) and indirectly (rate of parasitism). Because the scale insect only has one generation per year, where the first instar stage of the insect feeds upon leaf phloem tissue before over-wintering upon the bark and developing through the second instar stage prior to pupating and becoming adults the following spring, it was not expected for increased temperature to improve generational turnover rates, but simply enable females to lay more eggs / lay eggs that have a lower mortality rate. Furthermore, as the scale insect is very similar to an array of other insect species and genera, understanding how urban temperatures influenced its biology could give an indicator as to how other insect species would fare in similar conditions.
What the authors found, following a survey period, was that Parthenolecanium quercifex over-wintering in the second instar stage were 13x more abundant in warm locations than in cool ones (see the below figure). In addition to this, the ovisacs (where a single egg is layed) were 5.5x more abundant when deposited by the very same adult scale insects that were 13x more abundant, and these ovisacs gave way to first instar stage scale insects 7x greater than on trees in cold sites. Generations of scale insect that spent their entire life cycles in hot microclimates were also observed to, when placed in colder microclimates (in a greenhouse), still be found at higher abundances than scale insects that had spent their entire life cycle in cold microclimates. In this sense, scale insects may locally adapt (in a beneficial sense) to hotter locations, and it is suspected that the scale insect has this ability because populations are highly segmented and therefore site-specific adaptations can occur with relative ease (gene flow is ‘locked’ – meta-populations almost don’t exist beyond the level of but a single, or few, trees).
Conversely, no correlation was identified between urban temperatures and the rate of parasitism upon the scale insect. A total of six parasites were studied, of which none were found to have a significantly increased rate of parasitism when temperatures were higher. In fact, rates were near identical in hot and cold sites, as shown in the graph below. It is suggested that this is because the scale insect’s natural enemies are simply found in less abundance in urban locations, because of the poorer habitat quality. By a similar token, females were not found to lag more eggs on trees in hotter microclimates, and nor was host tree ‘quality’ deemed to impact upon the abundance of scale insects.
It was also mentioned that it is unlikely that stressed trees would be host to more scale insects, because scale insects would probably be found in lesser abundances where water and nutrients are lacking within the tree. Given urban trees typically struggle because of drought and a lack of nutrient availability, it is thus impropbable that tree quality is an influencing factor upon population levels of the scale insect. If it were then, because the scale insect is a sap-sucker that relishes nutrient-rich sap, a tree lacking this (because of drought and poor nutrient availability in the soil) would not be a able to support large numbers. Therefore, the increase in scale insects in hotter microclimates is likely to be independent of tree quality (condition).
To conclude, the authors remark that scale insects are more abundant in urban locations where the microclimate is warmer. As a consequence, if temperatures continue to warm in the urban setting, or become more homogenous (at a higher temperature) across a large urbanised spatial scale, scale insect populations may markedly increase and therefore be potentially very damaging for urban trees. Because urban trees are exposed to so many adverse conditions, an increase in pest activity is certainly not something that will help their case for survival. Such a weakened nature may also leave them exposed to other pests and diseases, which do rely upon weakened hosts to establish in great abundance.
Beyond the urban setting, if temperatures increase in rural locations, scale insects may also become more of an issue there. Granted, such rural locations are home to greater numbers of parasites (natural enemies), and thus an increase in numbers there may perhaps support an increase in parasitoid abundance as well. This is, however, just speculation.
Source: Meineke, E., Dunn, R., Sexton, J., & Frank, S. (2013) Urban warming drives insect pest abundance on street trees. PLoS One. 8 (3). p1-7.
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